Inspection instruments of the type under discussion here are capable of detecting cracks and other deformations is a workpiece by measuring the eddy current induced in the workpiece. The term eddy current, as generally understood in the art and as used herein, refers to a current created or induced in a conductor by an applied varying magnetic field, or by moving the conductor in a magnetic field.
Known instruments that utilize the eddy current effect generally include a probe having one or more probe coils intended to be positioned adjacent the workpiece. Although different probes will have different dimensions, by way of example, a probe may be about one-tenth of an inch in diameter and about three inches in length.
One type of probe, known in the art as a split core differential probe, has a pair of probe coils symmetrically spaced from a central axis. The probe is rotatably engaged to a small motor or the like for rotating the probe with respect to the axis. The probe coil leads extend through the motor, by way of slip rings or the like, and are connected to a cable which at its other end is connected to other circuit elements and to a signal interpretation unit. The probe coils form part of a measuring circuit, such as a bridge circuit, together with the aforesaid circuit elements.
Before a workpiece can be inspected, the instrument must be calibrated. With both probe coils positioned remote form any conductor, the motor rotates the probe and an AC drive signals is applied to the bridge circuit at a selected frequency. The output signal of the bridge circuit in this condition serves as the calibration signal.
The magnitude of the calibration signal can be adjusted by varying the impedance of the bridge arms, for example by substituting different circuit components. Since two of the bridge arms generally include a reactive impedance which varies with the frequency of the drive signal, the frequency of the applied drive signal must remain constant during calibration and inspection.
To inspect a workpiece, the probe is brought adjacent the latter and the probe coils are rotated at a constant rate. Thus, each probe coil will move proximate the workpiece and then away from it during one complete rotation. The magnetic field produced by each probe coil due to the combined effect of the applied AC drive signal and the coil movement induces an eddy current in the workpiece.
The magnetic field created by the eddy current interacts with the magnetic field of whichever probe coil is proximate the workpiece at a given instant. This interaction causes a change in the impedance of the proximate probe coil while the impendance of the other probe coil of the pair, i.e., the one remote from the workpiece, remains substantially at its calibration value. The bridge circuit therefore becomes unbalanced and the resultant output signal is representative of the magnitude of the eddy current. When the previously more remote probe coil moves to a position proximate the workpiece, the situation is reversed and the bridge circuit again becomes unbalanced. Thus, the bridge circuit becomes unbalanced twice in each rotation of the probe.
As the rotating probe is passed along the workpiece, cracks and other discontinuities of deformations in the workpiece will produce changes in the magnitude of the induced eddy current as compared to the magnitude of the induced eddy current in areas that do not have these anomalies. This results in corresponding variations in the magnitude of the impedance changes which the probe coils undergo during rotation and the impedance changes thus affect the bridge circuit output signal. Hence, the output signal, specifically the amplitude of the output signal variations, is a measure of the condition of the workpiece.
In the above-described instrument, the probe and the motor must be portable and they may have to operate at a considerable distance from the remaining bridge circuit elements mentioned above. The connecting cable therefore is of appreciable length and its capacitive reactance must be taken into account in adjusting, calibrating and operating the bridge circuit at the frequency of the applied AC signal.
Specifically, at the frequency of the drive signal, the capacitive reactance of the cable may resonate with the inductive reactance of the coils. If resonance does occur, the generated noise will deteriorate the signal-to-noise ratio of the bridge circuit output signal. Under these conditions, the point at which resonance occurs will be below the self-resonance frequency of the probe coils and it will thereby limit the operating frequency range of the bridge circuit.
Heretofore, the problems discussed above, particularly the problem of generated noise, were often attributed to poor coil construction and/or design and the suspect probes were discarded. That practice materially increased the cost of making the measurements without a commensurate increase in accuracy. Further, in practice it is not uncommon to use different lengths of cable as required by the particular workpiece. For example, when using existing instruments to inspect bolt holes in an aircraft engine, the signal interpretation unit generally is kept stationary and an operator walks around the engine with the rotating probe to inspect each bolt hole. Since it is preferable to use as short a cable as possible in order to reduce the reactance introduced by the cable, a shorter cable may be substituted for bolt holes located close to the instrument.
Since the connecting cable connects the probe coils to the remaining bridge circuit elements in the instruments described, the cable reactance becomes part of the bridge circuit impedance. Hence, the impedance of the bridge circuit components must be re-adjusted whenever cables of different length, and hence of different capacitive reactance, are exchanged in order to maintain a calibration signal of constant amplitude. Such a re-adjustment procedure can be time consuming and it presents serious constraints on the utility of present day inspection instruments in a busy work environment.